BAMS Paper Makes Case for Spectral Measurements

Published: 17 November 2020

ARM has more than 2 decades of spectral data sets available for use

Radiometers and spectrometers at Southern Great Plains
Radiometers and multiwavelength spectrometers at ARM’s Southern Great Plains atmospheric observatory gather information about solar energy, a large variable in the earth system equation.

A new Bulletin of the American Meteorological Society (BAMS) early online release describes the capabilities of the Atmospheric Radiation Measurement (ARM) user facility’s shortwave spectral radiometry and highlights recent uses of spectral measurements.

Ground-based shortwave sensors measure the ultraviolet, visible, and near-infrared spectral ranges that make up the solar energy spectrum. Earth absorbs and reflects this solar energy, and it emits infrared energy back into cold space. If the balance of energy coming to and leaving from Earth changes, this leads to changes in global temperature.

A critical piece of this radiation budget is the atmosphere, a thin layer filled with gases, aerosol, and clouds that all interact with and affect incoming and outgoing energy. Scientists must understand the processes in the atmosphere that drive changes in the radiation budget to predict the impact on Earth’s natural systems. (Learn more about atmospheric radiation.)

Laura Riihimaki, lead author of the BAMS paper, and her colleagues recognized the wealth of information in more than two decades of ARM spectral data sets. This information is crucial as scientists attempt to tackle challenges in representing earth systems in models.

A Maturing Data Set

The strength of ARM’s shortwave spectral capabilities lies in their placement with ground-based aerosol-observing and active remote-sensing instruments at fixed and mobile ARM sites. As the instruments work in tandem, information on radiative, aerosol, and cloud properties is gained in high time resolution. These observations inform scientists about processes such as cloud-aerosol interactions and their radiative effects, which introduce large uncertainties in earth system prediction.

For example, the BAMS paper summarizes a 2015 Atmospheric Measurement Techniques study that looked at precipitation formation in warm clouds. Researchers used vertically pointing radar and lidar measurements in concert with zenith-pointing spectral radiometer observations. ARM shortwave spectral measurements helped quantify cloud and precipitation properties, allowing for simultaneous retrievals of cloud and drizzle properties.

“We think that shortwave spectral radiation measurements are maturing into a data set that has the potential to address new sets of problems,” says Riihimaki, a research scientist at the Cooperative Institute for Research in Environmental Sciences/NOAA Global Monitoring Laboratory.

“It seemed like the right time to start to show the broader atmospheric science community some examples of what these measurements are capable of and hopefully spur some new innovative uses.”

Laura Riihimaki, lead author of a BAMS paper on ARM shortwave spectral radiometry capabilities

The BAMS paper came out of a February 2019 ARM shortwave spectral radiometry strategy review meeting, which Riihimaki organized with co-authors Connor Flynn and Allison McComiskey. The meeting discussed the state of the science and possible future applications of spectral measurements. (Read the meeting report.)

The authors are particularly excited about hyperspectral radiation measurements, which allow for continuous spectral measurements over a wide range of wavelengths. This is a change from previous narrowband measurements, which are limited to certain wavelengths.

Venturing Into New Territory

The paper lays out several recent uses of ARM’s spectral measurements and makes a case for more scientists to incorporate these key observations into their studies.

Cloud microphysical properties, such as ice or water phase, produce unique, known spectral signatures. At remote polar sites (e.g., ice sheets), observations are sparse and large instrumentation, such as aircraft and scanning radar, can be difficult to operate and maintain. Operating a shortwave spectral radiometer at these sites, just as ARM did in 2015 and 2016 during the ARM West Antarctic Radiation Experiment (AWARE), can give new insight into interactions between cloud optical properties and surface changes.

After years of evaluating the strengths and limitations of the instruments, says Riihimaki, “it seemed like the right time to start to show the broader atmospheric science community some examples of what these measurements are capable of and hopefully spur some new innovative uses.”

Read the BAMS paper.

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ARM is a DOE Office of Science user facility operated by nine DOE national laboratories.